Amplifier



FIG.

Dec. 16, 1941.

T. H. CR-ABTREE AMPLIFIER Filed NOV. 22, 1938 2 Sheets-Sheet 1 [NI/ENTOR -7'.H.CRABTREE BY ATTORNEY Patented Dec. 16, 1941 AMPLIFIER TheodoreH. Crabtree, New Brighton, N. Y., assignor to Bell TelephoneLaboratories, Incorporated, New York, N.;Y.,. a corporation of New YorkApplication November 22, 1938,' Serial No. 241,805

5 Claims.

This invention relates to amplifiers, particularly to amplifiers forsound recording systems and the object of the-invention is an amplifiercapable of use with-recorders of different types.

Many of the recorders commonly used require -merely that the drivingamplifiers shall be capable of providing the desired output level'and oftransmitting a wide band of frequencies-at uniform efliciency. Therequirements for an amplifier to drive recorders of the feedback typeare however much more stringent so that an amplifier for generalrecording purposes must be capable of being modified in severalimportant respects with minimum loss of time.

According to this invention, when the amplifier is being used to drive arecorder of the negative feedback type, one or more stages'may be usedexclusively for amplifying'the energy fed back to the input of theremaining stages on which the signals to be recorded are impressed.

In this v manner a very high degree of negative feedback is obtainedwith the result that the recorder may have a fiat response over a verywide range of frequencies and amplitudes.

Switching means or strapping connections are provided in variousportions of the circuit so that when the recorder to be driven is not ofthe feedback type, the feedback portion of the amplifier may be cut outor connected in such a manner as to increase the gain-of theamplifier.

When sufficient negative feedback is used to obtain a recordercharacteristic which is fiat over a very wide frequency range it is verydifficult to keep the phase shift around the feedback loop small enoughto prevent the feedback from becoming positive at high frequencies.According to a feature of this invention, in order to increase thestability of the amplifierat high frequencies, less feedback is usedthan is required'for pro ducing a fiat response down to the lowestfrequenciesin the band of interest. The low frequency deficiency is thencorrected by making the signal amplifier more efiicient in this portionof the range. This increased eificiency .is conveniently obtainedwithout additional circuit elements by a proper proportioning of thecoupling impedances between the feedback and signal amplifiers. 7

Alternatively, or as an additional precaution against instability,positivefeedback at frequencies above the band of interest may bedecreased or eliminated by shunting the feedback path with acondenser ofsuch small capacity as to have only slight-elfect on transmission withinthe band of interest but a relatively low impedance for the 55 higherfrequencies which tend to produce instability. I The output stage of theamplifier preferably comprises one or more self-biased pentodes of thebeam tube type. 'Applicant has found that with i circuits commonly usedfor these tubes the average screen current increases considerably withincreasing applied signalvoltage. The resulting increase in cathodecurrent increases the grid bias and reduces the plate current. Thisreduction of plate current not only reduces the-output capacity of theamplifier but also results in the tube operating on a portion of itscharacteristic where'the harmonic distortion is greater than it isforoperation at maximum plate current.

An important feature of this invention consists in so regulating thepotential applied to the screen'grids of the output tubes that thescreen currentvaries with the applied signal in such a manner that thecathode current, which is the sum of the screenand plate currents, issubstantiallythe same for all inputs up to full signal 'condition. 7

These and other features of the invention will 5 be more clearlyunderstood from'the following detailed description and the accompanyingdrawings'in which:

Fig.1 is a circuit diagram of an amplifier according to this invention;1

Fig. 2 shows-aportion of the circuit of Fig. 1 rearranged to illustratemore'clearly thefeature ofselectively amplifying the lower frequencies;and

Fig. 3 shows-the response curves of a typical feedback recorder.

- In Fig. lthe amplifier shown comprises a main amplifier ll operatingfrom recording bus-bars I2, I 3 which areenergized at a suitable levelfrom any desired input circuit such,'for example,- as a pick-upmicrophone l4-and preliminary. amplifier l5. The output of the amplifierII is impressed on the driving coil l6 of a disc'recorder ll which maybe of the type shown in Patent 2,161,489, granted to L. Vieth et a1.June 6, 1939.

The coil I6 of the recorder drives the-stylus 18 to cut arecordandsimultaneously generates in the coil l9 a voltage proportional to thevelocity of the stylus. This voltage is fed back over the conductors 20to the input of the negative feedback amplifier 2lthe output of which isimpressed on the input of amplifier ll along with the incoming signalsfrom the recording bus-bars l2, l3 and reduces the resonant response ofthe recorder to a fiat characteristic in the manner which is in generalnow well understood-in the art.

Coming now to a more detailed description of the circuit, the signalsfrom the recorder bus-bars are impressed on the primary of thetransformer 22 the divided secondaries of which are shunted by resistors23, 24 and 25, 26 respectively. The portions of the secondary potentialexisting across resistors 24 and 26 are impressed on the grids 21 and 28of the push-pull tubes 29 and 30 over the circuits including switch 3|,resistor 32, switch 33 and conductor 34 and switch 35, resistor 36,switch 31 and conductor 38 respectively.

The tubes 29 and 35 are self-biased by the in- 7 -again operating withreduced capacity and individual resistors 39, 40 and 4|, 42 andrals'oby' the common resistor 43, resistors 40, 4| and '43- being by-passedfor signal currents bythe' condenser 44. The outputs of tubes 29 and 33are resistance-capacity coupled in the well-known creased harmonicdistortion as in the case where th'efscreen potential regulation ispoor.

In 'applicants circuit the screen grid is at the potential of thepoint58 which is determined by the potential drop in resistor 59 due to thesum of the screen currents and the current in resistor manner to thebeam tubes 45 and 46 which are p self-biased by the common cathoderesistor '82, and the output of these latter tubes drives the recorderIT as already explained. Internal negative feedback connections, 41 and48 extending from the plates of tubes 45 and 45 to the oathodes of tubes29 and 30 to reduce the noise and distortion of the amplifier and toreduce the effective output impedance of the beam tubes to a value whichmore nearly matches the primary impedance of the output transformer 49in the manner well understood in the art.

Since at times other recording systems may be operating from thebus-bars I2 and 13 over circuits such as 58, it is necessary to preventthe negative feedback energy impressed on the inputs of the tubes 29 and30 from being reflected back through the transformer 22 to the bus-bars.This is accomplished by making the value of resistors 23 and 25 of theorder of ten times the value of resistors 24 and 26 so that thecircuitback to the bus-bars presents a high impedance to reflected currents.While this portion of the circuit also attenuates the incomingsignaLthat is not a serious objection in systems of the type shown'sincethe energy level at the bus-bars is ordinarily quite high. v

The amplifier is energized from the alternating current supply 5! bymeans of the; usual rectifier 52 and filter network 53, the tubefilament circuits (not shown) being connected to suitable taps on thewinding 54 of the transformer 55. It will be noted however that thescreen grids 55 and 51 of the beam tubes and 46 are connected to thepoint 58 between resistors 59 and 59 which comprise a bleeder circuitacross the output of the filter 53. The manner in which these resistorsare proportioned to give improved operation of the amplifier will now bedescribed. 7

Applicant has found that when beam tubes or other power pentodes areoperated with a given plate supply voltage into a given load impedance,there are optimum screen grid and cathode bias voltages which givemaximum power output and minimum harmonic distortion. In conventionalbeam tube circuits however, the screengrid voltage which largelydetermines the plate current, falls off so much at full signal that theplate current is materially reduced thereby decreasing the outputcapacity. and causing the tubes to operate at a point on the gridvoltage-plate current characteristic, where the distortion in the outputis considerably increased.

It is, of course, well known that the plate current in these tubesdepends on the screen voltage to a greater degree than on the platevoltage and the screen grid is therefore usually connected to thevoltage source through a resistor which limits 60. As the screen currentincreases with increasing signal the potential of point 58 will bedecreased due to the larger potential drop in resistor 59 and by properchoice of the values of resistors 59 and 60 the screen potential can bemade to vary in sucha manner that the increase in screen current from nosignal to full signal compensates for the decrease in plate currentthereby keeping the cathode current and grid bias for each tubeconstant.

In general this involves the use of resistors low enough to pass ableeder current which is large as compared with the screen current. .Forexample, when using beam tubes of one wellknown type with a platevoltage of about 400 volts and screen voltage of about 210 volts at nosignal, resistor 59 may be 5800 ohms, resistor may be 7000 ohms in whichcase the bleeder current i about 40 milliamperes and the total screencurrent of about 4 milliamperes. The procedure for determining theresistor values requiredin a given case is first to determine the gridbias necessary for the desired plate current at full signal with -agiven screen potential, then measure the cathode current and determinethe screen potential which gives the same cathode current at no signal.7 Having determined the difierences between the screen potentials andscreen currents at full signal and no signal, respectively, the value ofresistor 59 necessary to produce this variationis determined by dividingthe difference in the screen :Volttgfis by the difierence in the screencurrents for the twoconditions. The value of resistor 60 is then readilydetermined since it must be the same fraction of the total bleederresistance that the no signal screen voltage is of the output voltage ofthe filter. 7

An amplifier regulated in' this manner not only has higher maximumoutput'c-apacity and lower harmonic distortion than conventionalcircuits butit also has a further advantage particularly in circuitsusing negative feedback. When a negative feedback amplifier is energizedfrom a rectifierwhich has poor regulation," sudden changes in load onthe rectifier set up transients in the amplifier which cause blockingand moton-boating. The current supplied to the output stage of anamplifier comprises a verylarge proportion of the total load on therectifier but in this circuit the current to the output stage issubstantially constant. The output voltage' of the rectifier willtherefore vary onlyslightly for large changes in signal level and theamplifier is consequently much more stable in its operation.

In Fig. 3 of the drawingscurve It] represents a typical undampedresponse characteristic of a recorder of the type indicated at IT inFig. 1. It will be noted that such a recorder should be of the singlyresonant type preferably with the resonant peak occurringslightlyabovegeometric mean frequency :of the range .of interest. If, forexample, the recorder is to have a fiat response for frequencies from9000 cycles down to 30 cycles per second, this result may be obtained byusing sufiicient negative feedback to reduce the recorder response tothe line 6. .When such a large amount of feedback'is used .over such awide frequency range, however, it is very difiicult to prevent thefeedback from becoming positive for frequencies near and beyond theupper limit of the band and hence the recorder develops an irregularrising characteristic 62 and becomes unstable at these high frequencies.The resonant frequency of the recorder is usually fixed by otherconsiderations within the range of about 900 to 1000 cycles so that theamount of feedback used can usually be considerably less than the valuecorresponding to curve 6| without impairing the fiatresponse up to theupper limiting frequency of interest. For example, in the case shown theresponse may be reduced only to the line 63 which is fiat over the upperportion of the desired range but definitely drooping above the limitingfrequency and therefore much more stable. It will be seen, however, thatreducing the amount of feedback to insure stability at high frequenciesalso results in a drooping characteristic-over the lower frequencyrange,

such as between 80 and 30 cycles per second, which is part of thetransmission band.

In the circuit of Fig. 1 condensers 64 and 65 in the output of thefeedback amplifier 2| are of such capacity that their impedances for allfrequencies in the transmission band are low as compared with theimpedances of resistors 24, 32 and 26, 36 shunting the inputs to thetubes 29 and 30 of the main amplifier ll. Under this condition as iswell understood in the art, the transmission characteristic of thecoupling between the feedback amplifier 2| and the main amplifier IIwill be substantially fiat over the transmission band. The condensers 64and 65, however, as will be seen more clearly from Fig. 2, are alsoparts of shunt paths across the inputs of tubes 29 and 30 of amplifierll. Condenser 64, resistor 66 and condenser 61 of the rectifier filternetwork form a path from the grid 21 of tube 29 to the grounded cathodeand similarly condenser 65, resistor 68 and condenser 61 from a pathacross the input of tube 30.

Condensers 64 and 65 .will ordinarily be of the order of .02 microfaradand condenser 61 of the order of 8 microfarads so that the impedance ofcondenser 61 is negligible as compared with the other elements of theshunt paths. A signal impressed on the primary of transformer 22 willproduce a potential across resistors 24 and 26 and these potentials areimpressed on the shunt paths referred to above through resistors 32 and36, respectively. Since the impedance of the condensers 64 and 65increases with decreasing frequency the proportion of th signalpotential applied to the grids of the tubes 29 and 30 will beprogressively greater as the frequency decreases and the signal pathwill have a rising low frequency characteristic as indicated by thecurve 69 in Fig. 3.

By proper choice of the values of the condenser and resistors thisincrease in the efiiciency of transmission may be made of the propermagniture to compensate substantially for the drooping characteristic ofthe undamped recorder at thes frequencies so that as shown the flatresponse withnfeedbackisf extended down to the lowest frequency-of"interest. It will be clear from the foregoing explanation that thisrequired correction at low frequenciesxmay, be obtained withoutimpairing the feedback Ecircuit transmission characteristic byusingcoupling condensers 64 and 65 of such capacity that theirimpedances over thefrequencyrange in which compensation is required isgreater than the impedance of resistor 66 shunting the feedbackamplifier to give. increased. signal transmission at low frequencies butless thanthe impedance of resistors '32 and 24 to keep the feedbacktransmission substantially constant.

If for any reason'it is impractical to make the amplifier sufficientlystable at high frequencies by reducing the amount of negative feedbackusedand compensating for a drooping characteristic in. the manner justdescribed, the resistors ll, 12 in the input of the feedback amplifiermay be shunted by condensers l3, 14 of the order of .001 mfd. to .006mfd. capacity. Such condensers are of such high impedance as to have avery small effect on transmission for frequencies within the band ofinterest but 10w enough in impedance, for frequencies above thetransmission band where the feedback is posi tive, to reducetransmission around the feed back loop to a level which does not maketheamplifier unstable. 7

When the recorder to be driven is not of the feedback type the feedbackamplifier may be readily disconnected by operating switches 33, 3|,-35.and 31 to their dotted positions. In this case energy cannot be fed backto the bus-bars and the. fullpotential across the secondaries oftransformer 22 may therefore be impressed directly on the grids of tubes29 and 30 through the switches 33 and 31. A part or all of the increasedover-all gain due to the elimination of external feedback and to the useof the full voltage across the secondaries of transformer 22 may beutilized, if desired, to reduce distortion by increasing the internalnegative feedback in the amplifier ll. The switch 10 in the positionshown connects the cathode resistors 39 and 42 to ground throughcondenser 44 thereby'by-passing theisignal currents around resistors 40,4| and 43 and leaving only resistors 39 and 42 effective to impressnegativesfeedback voltages on I the gridsof tubes 29- andn30. By closingthe switch 10.0n contact8l only resistor '43 is bypassed .and .thealternating potentials developed across resistors 40 and 4| are alsoimpressed on the grids of tubes 29 and 30 thereby increasing thenegative feedback and correspondingly reducing the over-all noise anddistortion.

When the recorder used is not of the feedback type and more gain istained satisfactorily in the amplifier I I, the feedback amplifier 2|may be used in the signal circuit. In this case switches 33 and 31 areleft in the positions shown, switches 3|, 35 and 15 to 18 are moved totheir dotted positions and switch 10 is operated to either position inaccordance with the gain and distortion requirements of the particularcase as described above. Signals applied to the transformer 22 are thenfed over conductors 19, to the input of amplifier I I through switches33 and 31.

It will therefore be seen that this amplifier is well adapted for use asa general purpose recording amplifier and that the circuit may bereadily changed to meet the requirements of recorders of different typeWhile the invenrequired than can be obtion has beendescribed for purposeof illustration with reference to a particular circuit it will beunderstood that it may be modified in various ways within the scope ofthe following claims.

What is claimed is: V 1. In an amplifying system, a source .of signals,a resonant receiving device, a signal amplifier connecting the source tothe device, means for generating a voltage proportional to the responseof the device, a feedback circuit including an amplifier for impressingthe voltage on the input of the amplifier to make the response of thedevice uniform over a predetermined frequency range and a. condensercoupling the feedback amplifier to the signal amplifier and formingwiththe output of the feedback amplifier a shunt across the input of thesignal amplifier, the capacity of the, condenser being proportioned withrespect to the impedance of the input of the signal amplifier and theoutput of the feedback amplifier so as to give a fiat frequencytransmission characteristic in the feedback circuit and a non-linearfrequency transmission characteristic through the signal amplifier tocompensate for the characteristic of the receiving device below thefrequency range in which the response is made uniform by the feedbackcircuit.

2. An amplifying system comprising a source of signal currents, areceiving device, a signal amplifier having an input circuit connectedto said source and an output. circuit connected to the receiving device,a second amplifier having an input circuit and an output circuitconnected to the input of the signal amplifier, means. for selectivelyconnecting the input of the second amplifier either to the receivingdevice to feed back to the signal amplifier a voltage proportional tothe response of. the device to reduce the distortion in the output ofthe device or between the source of signal currents and the signalamplifierto increase the gain of the system, and means for, producingnegative feedback in the signal amplifier to utilize the increased gainof the system to reduce the distortion in the signal amplifier.

3. In an amplifying system, a source of current, a resonant receivingdevice, an amplifier having an input circuit connected to the source andan output circuit connected to the device, means associated with thedevice for generating electric currents of an amplitude varying with theresponse of the device, a negative feedback circuit connecting saidmeans to the input of the amplifier for reducing the resonant responseof the device-to a fiat characteristic over a predetermined frequencyrange, said circuit including reactive elementslof such value that thefeedback becomesgpositive for frequencies above said range and tends-to.cause instability in the operation of the amplifier, anda condenserconnected across; the feedback circuit of such small capacity as to haveonly slight effect on the transmission offrequencies within the range ofreson'ant response of the device, but to substantiallyreduce-transmission of. the frequencies tending to cause instability.

4-. In an amplifying system, a-source of signal currents, a resonantreceiving device, an amplifier having an input circuit, connected to thesource and an, output circuit connected to the device,'means associatedwith the device for generating electric currents of anamplitude varyingwith the response-of the device, a circuit conducting thegeneratedcurrents to the input of the amplifier in phase opposition to the signalcurrents and producing the maximum negative feedback action consistentwith stable operation whereby the resonant response of the device isreduced to a substantially fiat characteristic over a frequency rangeextending from a frequency within the band to be transmitted up to theupperlimit of the ,band, means in said circuit for reducing transmissionfor frequencies above said range and, coupling elements between thecircuit and the amplifier having a fiat transmission characteristic forthe generated currents for selectively increasing the transmissionthrough the amplifier for currents fromthe source of frequencies belowsaid range and producing a fiat characteristic down to the lower limitof the band.

5. In an amplifying system, a source of signal currents, bus-barsconnected to said source for distributing the currents, a signalamplifier having input and output circuits, a connecting circuit betweenthe bus-bars and the input circuit and a feedback circuitconnected tothe input circuit and carrying currents varying in amplitude with theoutput of the amplifier, said connecting circuit having a shuntimpedance and a high series impedanceconnecting a small portion of theshunt impedance across the input of the amplifier, whereby saidconnecting circuit presents a high impedance between the feedbackcircuit and the bus-bars.

THEODORE H. CRABTREE.

